The present invention relates to gas discharge display tubes and more particularly to gas discharge display tubes of the type which include a plurality of display cathodes which may be selectively enabled to form a plurality of display patterns.
Gas discharge display tubes of the foregoing type include an anode and a plurality of display cathodes spaced therefrom. The display cathodes are separated from the anode by an insulating gas which permits current flow between the anode and cathode only when the potential difference therebetween exceeds the breakdown voltage of the gas. When this potential difference (the turn-on voltage of the tube) is exceeded, current will flow through the display cathodes and they will light. By adjusting a potential difference between each cathode and the anode independently, it is possible to illuminate only selected cathodes so as to display a desired pattern. The luminosity of each of the enabled cathodes is determined by the magnitude of the current flowing through the cathode. By adjusting current flow through each illuminated cathode, the luminosity of that cathode may also be controlled.
In prior art gas discharge display tubes, the foregoing results are obtained by placing a predetermined voltage on the anode of the discharge tube and adjusting the voltage at the display cathodes to selectively turn on desired cathodes. Those cathodes which are to remain off are biased at a potential near the anode potential while those cathodes which are to be turned on are switched to a voltage at least the turn-on voltage below the anode voltage. Once the given cathode is turned on, a current limiter circuit associated with the respective cathode controls the cathode current to ensure the desired luminosity.
While the foregoing arrangement is generally satisfactory, it cannot economically be used in connection with low cost bipolar or MOS integrated circuitry. Since such circuitry is capable of outputting relatively low voltage swings (typically below 30 volts), their output cannot be directly used to drive the cathodes of the display tubes. Additionally, the relatively low cost of the bipolar or MOS control circuitry used to turn selected cathodes on is offset by the fact that individual current control elements must be utilized in connection with each individual cathode. As a result of the foregoing drawbacks, the use of gas discharge display tubes has generally been rejected in connection with low cost bipolar or MOS circuit arrangements despite the generally advantageous characteristics of gas discharge tubes: relatively low power for large digit size, the lowest cost for custom patterns and patterns where both size and number of digits is relatively large.